I'm not a graphics person that can make this... but in case someone who is thinks it's a great idea: :)
It occurs to me that a cut-and-fill system might be cool to play with.
Method:
1. Elevated road that looks like road-on-gr**** and takes up full tile width like a road on gr**** would (or road on sidewalk might be even better... or both)
2. Bridge that also looks the same.
Way it would be used:
1. Use slope tools to lower ground (probably in cities, where existing road is)
2. Build train tracks on lowered tiles
3. Build elevated road on top
3. Make connections by building a sloped tile and bridging to the elevated road (but this single-tile bridge will be level)
Does that make sense? And does it sound cool?
If train tracks were also made elevated this way, could have road under tracks...
Doesn't a tunnel do what you want already?
Well, for one: entrances would take one tile instead of two (just a downslope instead of a down slope + tunnel entrance)...
It also might be interesting to have two-tiles-wide systems of rails/road... or even double-stacks of road for express vs. local....
*shrug*
Just throwing it out there, at least :)
It also occurred to me that I might be able to give it a go by using some elevated road as a guide, although currently I don't have sources to any elevated road (that I'm aware of)...
Didn't someone already do this? I remember on the old forum, bridges which looked like ground tiles so you could put curved "tunnels" underneath (possibly for subway?) This was before the underground mode was implemented...
I can't remember who it was now. Still, this is entirely possible right now with the right graphics :)
IIRC Fabio Gonella had done something similar.
Also, I think we will need bridges between two vertical "slopes".